Solar heat gain through glazing and how to control it

Glazing is the most powerful thermal element in most commercial envelopes. A window admits daylight and views, but it also admits solar radiation that becomes heat inside the building. In perimeter zones of glass-heavy offices, solar heat gain is often the largest single cooling load and the main cause of comfort complaints.

The good news is that solar gain is highly controllable. Glazing selection, shading and orientation can change perimeter loads dramatically, often for less cost than the HVAC capacity needed to deal with the heat. This guide explains the physics, the ratings, and the practical options.

How solar heat enters

Solar radiation reaching a window is either reflected, absorbed by the glass, or transmitted directly into the room.

• Transmitted radiation passes through and is absorbed by floors, furniture and walls, which then heat the air.
• Absorbed radiation heats the glass, and part of that heat flows inward by convection and radiation.
• Reflected radiation leaves the building.

The total solar heat entering is the transmitted portion plus the inward-flowing part of the absorbed portion. This is captured by a single number: the solar heat gain coefficient.

Separately, heat is also conducted through the glazing because of the indoor to outdoor temperature difference. That conduction is described by the U-value and is a separate component of the load.

Window performance ratings

Always use whole-window values that include frame and edge effects, rated according to recognized procedures such as NFRC 100 and NFRC 200 or CSA A440.2, rather than centre-of-glass values. Centre-of-glass numbers look better on paper but do not represent the installed window.

Solar gain in load calculations

The solar component of the cooling load depends on:

• Orientation and the sun's position for the design month and hour.
• Glazing area and whole-window SHGC.
• Interior and exterior shading devices.
• Building mass, which delays the radiant portion.

East glazing peaks in the morning, west glazing in late afternoon, and south glazing in Canada can peak in late summer or early autumn when the sun is lower. Horizontal skylights receive the most intense summer sun. Because each orientation peaks at a different time, perimeter zones should be separated by exposure; see zoning a commercial building for HVAC.

Controlling solar gain

Glazing selection

Spectrally selective low-e coatings transmit much of the visible spectrum while blocking much of the near-infrared, which carries a large share of solar heat. This allows a relatively high VT with a low SHGC. Tinted glass also reduces SHGC but cuts daylight more and increases glass temperature, which can affect comfort near the window.

Triple glazing, now increasingly common in Canada for its lower U-value, can be specified with different coatings on different surfaces to tune SHGC for each orientation.

Exterior shading

Exterior shading intercepts sunlight before it reaches the glass, making it the most effective control.

• Horizontal overhangs and fins work well on south facades, where the summer sun is high.
• Vertical fins help on east and west facades, where the sun is low.
• Exterior blinds and screens can be very effective but require maintenance and must withstand wind and snow loads.

Interior shading

Interior blinds and roller shades control glare and reduce some solar gain, but because the heat has already passed through the glass, they are much less effective than exterior devices. Light-coloured, reflective interior shades perform better than dark ones.

Dynamic glazing

Electrochromic and other switchable glazing changes its SHGC and VT in response to control signals. It can reduce peak loads and glare without blocking views, at a higher capital cost.

Window area

The most direct control is glazing area. NECB limits the fenestration and door to wall ratio for the prescriptive path, with the allowance decreasing as heating degree-days increase, from about 40% in milder climates down to about 20% in the coldest. Projects exceeding this must comply by trade-off or performance path. See NECB 2020 and HVAC compliance.

Comfort near windows

Solar gain affects comfort beyond air temperature. Direct sun on occupants and hot glass surfaces raise the mean radiant temperature, so people near windows can feel too warm even when the thermostat is satisfied. In winter, cold glass causes radiant discomfort and downdrafts. ASHRAE Standard 55 addresses these effects through operative temperature and radiant asymmetry limits; see thermal comfort and ASHRAE Standard 55.

Balancing heating and cooling in Canada

Canada's long heating season complicates glazing choices. A low SHGC reduces summer cooling but also reduces useful winter solar gain. A sensible approach:

1. Model the building's annual energy use with several glazing options rather than optimizing only for peak cooling.
2. Use lower SHGC on east and west facades where gains are hard to control and peaks are high.
3. Consider moderate SHGC on south facades with exterior overhangs that block high summer sun but admit low winter sun.
4. Prioritize low U-values everywhere, since conduction losses matter throughout the heating season.
5. Check daylight levels and glare along with thermal performance.

Energy modelling provides the evidence for these trade-offs, and it is usually required for performance path compliance anyway.

Frequently asked questions

Is a lower SHGC always better?

Not in every Canadian building. Lower SHGC reduces cooling but also reduces free solar heat in winter. The right value depends on orientation, climate, building type and internal gains, and is best chosen with energy modelling.

Do interior blinds reduce cooling loads?

Somewhat, and they are valuable for glare control. Because the heat has already passed through the glass, they are far less effective than exterior shading or low-SHGC glazing.

What is the difference between centre-of-glass and whole-window values?

Centre-of-glass values describe only the middle of the glazing unit. Whole-window values include the frame, spacers and edges, which usually perform worse. Use whole-window values for load calculations and code compliance.

Key takeaways

• Solar heat gain is often the largest perimeter cooling load in commercial buildings.
• SHGC, VT and U-value describe different aspects of window performance; use whole-window ratings.
• Exterior shading is the most effective control; interior shading mainly helps with glare.
• Each orientation peaks at a different time, so zone perimeter spaces by exposure.
• In Canada, balance summer cooling against winter heating with energy modelling.